ASUS F550Z no backlight

[erbuzzicker]

Hello everyone,

laptop: ASUS F550Z
motherboard: X550ZE rev. 2.0

laptop turns on but there is no backlight on display but I can see image if I use a torch on the display.
Unfortunately, schematic seems not to be available and I only have boardview.

I have checked LVDS connector and there is no 19v on pins 39 and 40 AC INV. 19v is present at pin 2 of B4501 but it's 0v on pin 1, which is connected only to 2 components (R4563 and C4509) as you can see.

Now my problem is to understand if it's a issue related to the motherboard, LCD cable or display (but I know that, in case of no backlight, display usually can be excluded from the faulty reasons).

One thing you should know is that when I received laptop, B4501 was not present (maybe someone removed it to diagnose the problem) and it seems that something burned there because I found black track just there. I cleaned area and I put a new component from a scratch board (even if I don't know specs of that component that seems to be inductor).

Please help me to understand.

Thanks

[RethoricalCheese]

Re: ASUS F550Z no backlight

There seems to be a fuse F4501 on the other side of the board. Check that.

[erbuzzicker]

Re: ASUS F550Z no backlight

Hello, thanks for your interest. I have not mentioned F4501 because it is not present on motherboard

[electrocute]

Re: ASUS F550Z no backlight

Hello. I've not much experience but have you checked the LCD_BACKOFF#
and BACK_EN nets?

[erbuzzicker]

Re: ASUS F550Z no backlight

Hello. I've not much experience but have you checked the LCD_BACKOFF#
and BACK_EN nets?

Hello. No I have not checked them. I think that if problem is related to LCD_BACKOFF# or BACK_EN, I should not have 19v on pin 2 of B4501 but I am not sure of this. I'll check those 2 pins and let you know, thanks

[erbuzzicker]

Re: ASUS F550Z no backlight

BACK_EN is 3.3v so it's ok.

LCD_BACKOFF# and LID_SW# are both 3.3v on D4502 as well so they are also ok.

Any new idea please?

[erbuzzicker]

Re: ASUS F550Z no backlight

In the image attached, you can see B4501 I have soldered on MB because it was missing when I received the board. As you can see there's a burnt area under it on the motherboard track. Do you think it could be the issue of missing 19v on the output pin of B4501?

[piernov]

Re: ASUS F550Z no backlight

Left side isn't soldered properly at least. Also be 100% sure the component is an inductor not a capacitor (hard to tell from the picture).

[erbuzzicker]

Re: ASUS F550Z no backlight

Left side isn't soldered properly at least. Also be 100% sure the component is an inductor not a capacitor (hard to tell from the picture).

Thanks for your help. First, I am not sure what B4501 is because when I received the motherboard, it was not present and the area was burnt (I cleaned with alcohol).

So how do I know what it is just having only boardview? About the component you see in the picture, I have taken it from a donor board after have seen it was present really close to 19v pin of LVDS motherboard connector, so I thought it was a component of same type of the missing one.

[RethoricalCheese]

Re: ASUS F550Z no backlight

Looks like assuming has made you put a capacitor where it doesn't belong

Not sure what designator B would stand for but.. I'd blindly just replace it with a fuse or solder bridge if it's the only thing stopping 19V from going to the LCD connector.

[piernov]

Re: ASUS F550Z no backlight

B could be just a shorthand for FB = ferrite bead.

https://www.badcaps.net/forum/showpo...02&postcount=2

2.6. Never randomly bridge MOSFETs/fuse/components
Bridging components can easily lead to the destruction of the board without any way to repair it.
If it's a fuse, replace it with a matching fuse. If you don't, it will not be able to protect the board and if the source of the problem is still there, some other component will go up in flames.

[erbuzzicker]

Re: ASUS F550Z no backlight

Ok I will put a fuse or a solder bridge just to know if it solve my issue. Later I will try to find the right component to the missing one.

I'll let you know, thanks

[RethoricalCheese]

Re: ASUS F550Z no backlight

Oh yeah, you haven't checked resistance on backlight pins yet? Do that first, before bridging anything.

[erbuzzicker]

Re: ASUS F550Z no backlight

Oh yeah, you haven't checked resistance on backlight pins yet? Do that first, before bridging anything.

B4501 is connected to pins 39 and 40 (AC INV) of lvds connector. They should have 19v. Do you mean to check resistance to those pins? Which value should I have to be sure bridging will not cause any problem?

[erbuzzicker]

Re: ASUS F550Z no backlight

Resistance on pins 39 and 40 is 0 ohms if LVDS cable is connected to motherboard, 0L if LVDS cable is not connected to motherboard. Since LVDS cable is not connected, on other hand, to the display, can I assume that maybe the problem is LVDS faulty cable?

[RethoricalCheese]

Re: ASUS F550Z no backlight

Yes, either cable or LCD is faulty then and burned out that inductor. So first be sure to remove that short by replacing faulty cable (or screen) and then you can test by shorting/replacing. Otherwise you will just short out main power rail.